1. Field of the Invention
The field of the present invention relates to collision determining circuits designed to produce a starting signal for operating an air bag system or a belt retracting system upon collision of a vehicle.
2. Description of the Prior Art
In an air bag system for protecting an occupant from collateral injury upon collision of a vehicle, if only an acceleration signal produced by an acceleration sensor is used as a starting signal of the air bag system, there is a possibility that the starting signal may be output when an area of a vehicle body housing the acceleration sensor collides against a small object which would cause almost no damage to a vehicle body. Therefore, a means for preventing such a situation is required. In addition, it is necessary to operate the air bag system, not only upon a collision providing a large shock but also upon a collision providing a shock which is small but continued for a long time. In such a case, however, there is a possibility that the acceleration sensor would not output the starting signal and hence, a means for covering this situation is also required.
To avoid such a problem, a technique has been proposed (see Japanese Patent Publication Kokoku No. 8574/84) in which a speed found by integration of an acceleration produced by the acceleration sensor, i.e., a passenger inertial speed of an occupant moved, for example, toward a steering wheel by inertia due to a collision is calculated, and when such a calculated passenger inertial speed exceeds a predetermined reference value, a starting signal for an air bag system is output.
However, it is experimentally know that such a technique is effective when a front portion of a vehicle collides vertically against an obstacle, but when the front portion of the vehicle collides obliquely against an obstacle, a passenger inertial speed is not sufficiently detected even if the speed at the time of the collision is relative large and thus, there is a possibility of the starting signal not being produced.